1. Field of the Invention
The present invention relates to radiation detectors, and more specifically, it relates to high-performance, room temperature gamma-ray detectors.
2. Description of Related Art
Presently, semiconductor radiation detectors serve a crucial role in detecting illicit nuclear weapons and radiological dispersal devices (RDDs) by virtue of their ability to distinguish isotopes. The best performer in this arena is the germanium (Ge) detector which has resolution of 0.2%, although they do require cooling to cryogenic temperature to function properly (<110 degrees K). Ge detectors are large, >6 cm in length, and consequently are sensitive. Nevertheless, the cooling requirement increases power consumption. Cool down takes about an hour and the battery must be replenished after ˜8 hours, requiring logistical support for operations. Consequently, there has been an enormous effort to develop CZT (CdZnTe) as a RT alternative to Ge, mainly in terms of perfecting the crystal growth. While there has been substantive progress in CZT detectors, mastering the complexities of crystal growth and device fabrication has proved elusive. The effort has yielded typical detectors limited to ˜2% resolution, while <0.5% is desired to reduce false alarms by isotope identification. Moreover, it has proved extremely difficult to produce single-crystal material greater than 1.5 cm in size, and the poor yield from crystal boules is expected to impede the widespread fielding of these devices. Currently, the desire to identify and develop a RT detector with 0.5% resolution is paramount in detection scenarios in order to adequately distinguish between isotopes, as is the need for a semiconductor that is larger (for higher efficiency), more available, and lower cost.
The applications of RT semiconductor radiation detectors are extensive, involving handheld devices for primary and secondary inspection of packages and cargo containers, as well as simple pager-type devices that can hang on the belts of government workers in a ubiquitous deployment strategy (i.e., on the persons of police, postal workers, etc., wherein the detection of dangerous isotopes is transmitted to a central information processing computer).
The utility of a compact, inexpensive, sensitive, high-resolution RT radiation detector is enabling and can barely be overstated since they will be used in all ports-of-entry (shipping, airports, borders), as well as throughout cities and for entry into buildings and special events. Moreover, they would be used in military operations in foreign lands and for routine monitoring. Government agencies with immediate compelling needs include the Departments of Energy and Defense, Homeland Security, Intelligence Agencies, Emergency Response, and the Coast Guard. Other agencies such as DARPA, DTRA, and NASA are also interested in robust semiconductor radiation detectors for military and scientific purposes. It could be stated that the necessity of this particular device is one of the most urgent matters facing us in the detection of nuclear weapons and RDDs.
Many semiconductor candidates have been considered for use as room temperature (RT) radiation detectors. The semiconductor properties used to characterize RT radiation detectors include bandgap energy, melting point, maximum atomic number, resiliency and growth parameters. The bandgap energy (EGAP) should be >1.4 eV to allow near-room temperature operation with high resistivity, and should be <2.0 eV for adequate carrier mobilities. Thus, EGAP should be within a range from 1.4 eV to 2.0 eV. The melting point of the material should be within a range of >600 degrees C. to <1200 degrees C. A melting point (TMP) of <1200 degrees C. is desirable for ease of growth and >600 degrees C. for strength. The maximum atomic number, ZMAX should be >50 to provide adequate stopping power. The material should have low defect density for long carrier lifetime τCAR. The material should be non-hygroscopic and chemically and mechanically resilient to allow polishing, etching and lithography and cracking and evaporation of the material during growth should have manageable
Table 1 below shows known materials with favorable band gaps. CZT (CdZnTe) meets most, but not all, of the preferred properties, and is currently the most promising RT semiconductor detector material. Its development and implementation, however, has been hampered by difficulty in simultaneously achieving high resistivity and long carrier lifetime. Telluride precipitates have been implicated as the recombination centers that shorten the carrier lifetime.
It has been suggested that AlSb potentially offers better fundamental properties, particularly because of the favorable bandgap and mobilities. Its hygroscopic nature has complicated the development of electrical contacts, however. The crystal also tends to grow with multiple domains and loses Sb as vapor during growth. CMT is another new material that has not yet been developed but has substantial promise. GaAs has essentially been abandoned because the carrier trapping is excessive. HgI2 has a very high Z (high stopping power) but has been found to be extremely difficult and expensive to grow in large size. Ge exhibits ideal detection properties but demands cryogenic cooling to compensate for the small bandgap (in order to reduce the carrier population for high resistivity).
TABLE 1Comparison of semiconductor properties of potentialRT radiation detector materialsEGAPTMPMobility (cm2/Vs),Semiconductor(eV)(° C.)ZMAXCarrier Lifetimehole/electronHandlingCdZnTe1.6107752Precipitates trap 50/1000Fragile(CZT)electronsCdMnTe1.6107552May be better than  Low/~1000Fragile(CMT)CZTAlSb1.6106551Trapping observed200/420HygroscopicGaAs1.4123733EL2 trapping4000/8500GoodHgI22.126080~2 microseconds 4/100Very difficultto growGe0.6793832>2 μsec3900/1900Excellent;Requirescryogeniccooling